If I only use 3 notes, is that copyright infringement?

There are rumors that sampling only four notes is not copyright infringement because it is protected as "fair use". This notion of reducing copyright infringement down to the number of notes uses, however, is simply wrong. If you sample a single note, beat, or line from a sound recording without permission, that constitutes copyright infringement. Under current US copyright law, unauthorized "sampling" - no matter how minimal or seemingly innocuous- is usually not considered "fair use".

Under US Copyright law, the true test for copyright infringement is not the number of notes sampled, but whether the sample is "substantially similar" to the original work. The other main questions is whether it should qualify as "fair use".

In short, if you engage in unauthorized sampling and get sued by the owners, don’t expect to prevail in court on a "fair use" defense if you use the songs commercially for your own private benefit.